In many applications, the spars or beams of an aircraft structure have an H-shaped cross section. For some specific applications, the height of the spar or beam varies in certain areas along its length, in such a way that the upper or lower flange has a non-rectilinear profile. More typically, the lower flange has to be raised in a given area, and consequently the web is locally shorter, while the shape and dimensions of the upper flange remain unchanged.
For some of these applications, for example in the case of floor beams for passenger aircraft, it is particularly important to ensure the geometric precision of the beam. Above all it is necessary to ensure the flatness of the lower flange of the H-section, since this is attached to the floor elements and the web by means of which the beam is connected to the frames of the fuselage.
Polymerization tools, also known as “matched moulds”, are currently used for the manufacture of spars from composite materials, these tools being supports which can give the spar its final shape by means of the simultaneous application of pressure and heat in an autoclave. The curing tool has the task of supporting and containing the spar during these steps. The current technology is based on the use of metal mould and counter-mould tools which completely surround the component and have projections and recesses for producing the aforesaid local variations. Generally, these rigid tools are made from Invar and comprise an upper forming tool, a lower forming tool, and two lateral tools. The spar to be cured is positioned on one of the metal elements and enclosed by the other movable disassemblable elements of the equipment. All of these elements are then enclosed in what is known as a “vacuum bag”, made from polyamide (nylon) film to which a vacuum is applied. This assembly is then placed in an autoclave where a combination of heat and pressure is applied with a predetermined variation over time. The pressure applied to the vacuum bag is discharged on to the metal parts of the tools, which in turn transmit the pressure to the spar. As a result of this compaction, the pressure, combined with the simultaneous temperature increase, consolidates and cures the resin.
If the components of prior art curing tools are not perfectly connected, or if their geometry does not precisely match the beam to be produced, it is possible that non-uniform pressures, higher in certain areas and lower in others, will be applied to the uncured composite material. This means that the areas of higher pressure will contain less resin, with a reduction in the local thickness of the spar, while the areas of lower pressure may be poorly compacted, possibly leading to porosity of the finished spar. This problem is particularly critical in the case of spars of variable thickness. An imperfect joint between the edge of the spar to be cured and the edge of the tool also creates empty spaces into which the resin tends to flow under pressure, thus decreasing the amount of resin in the spar and creating undesirable reductions in thickness.
U.S. Pat. No. 5,454,895, in the name of one of the present inventors, describes an apparatus and a method for manufacturing a composite box structure with an upper skin joined to a lower skin by a series of parallel spars, each formed by the joining of two opposing C-section elements. In this method, longitudinally aligned inserts are provided and are wrapped in a series of layers, namely separating films, a closed bag, and one or more breathing layers. One lower skin layer, made from fibre-reinforced curable thermosetting material (or “composite material”), is positioned on a base plate of a forming mould; the C-shaped elements of the preformed spars of the same composite material are arranged around the inserts which are wrapped as described above, and the whole assembly is then positioned on the lower skin. An upper skin of composite material is then applied above the preformed spars, and the upper plate of the forming tool is positioned above the upper skin. All of these elements are enclosed in a vacuum bag. During the step of curing in an autoclave, pressure is applied to the outer surfaces of the upper and lower skins and to the lower flanges of the spars so as to compact them against the corresponding plates of the tool, while the webs of the spars are compacted by the adjacent internal bags. Thus the inserts impart a shape to the internal tubular bags in such a way that this shape is as close as possible to the final shape of the cavity which is to be obtained. This is done in order to avoid the formation of links which could lead to the breaking of the bag during pressurization and thus create areas of increased radius. In the configuration described in U.S. Pat. No. 5,454,895, the inserts also serve to restrict the deformation of the assembly formed by each of the pairs of adjacent internal tubular bags and the webs of the spars enclosed by them. This is because, when the resin is fluidized by heat and the web of the spar could assume any shape, the walls of the adjacent inserts constrict the membrane formed by the adjacent bags and force the web of the spar to remain flat.